The prevention of recombination by some form of reproductive isolation
(Romanes 1886; J. Linn. Soc. 19,
337-411) is necessary to initiate and preserve both
sexual phenotypic differences within species, and
phenotypic differences between species.

For species preservation isolating barriers are prezygotic (transfer barrier), or
postzygotic (developmental and sterility barriers). The latter occurs at the chromosomal
level (hybrid sterility due to impaired homology search), is likely to be the first
barrier to appear, and is finally manifest in both
sexes.

For sexual differentiation, the first and only barrier is at the chromosomal level, and
always applies to one sex (i.e. the
homogametic sex has similar sex chromosomes so there is no need to prevent recombination).

The heterogametic sex has different sex chromosomes and for regions
containing genes affecting sexual differentiation there must be the equivalent of the
hybrid sterility observed when members of allied species cross. The heteromorphic
chromosomes seen in the heterogametic sex probably evolved from homomorphic chromosomes,
where reproductive isolation initiated.

The antirecombinational effect of (C+G)%
divergence proposed for incipient speciation (Forsdyke 1996; J.
Theor. Biol. 178, 405-417) is applicable, in
principle, to incipient sexual differentiation. As with speciation, subsequent chromosomal
macromutations (e.g. segment deletions) might have substituted for the (C+G)%
divergence, so that no trace of the original mechanism of reproductive isolation would be
evident in many modern species. However, a step towards
speciation would have been taken in the heterogametic sex.

A corollary of this is that incipient speciation, manifest as
some degree of hybrid sterility when "varieties"
are crossed, should appear at the earliest stage in the heterogametic sex, even in genera
with homomorphic sex chromosomes (Haldane's rule for hybrid sterility). Later the
differentiation would extend to autosomes, and reproductive isolation between the "varieties" (then by definition "species")
would be complete. A proposed mechanism for Haldane's rule for hybrid inviability depends
on differences in dosage compensation (Forsdyke 1995; J.
Theor. Biol. 172. 335-345), and should not apply to
species with homomorphic sex chromosomes (Presgraves & Orr 1998; Science282, 952-954). For more (Click Here).